title: The AFB4 auxin receptor is a negative regulator of auxin signaling in seedlings.
abstract: The plant hormone auxin is perceived by a family of F box proteins called the TIR1/auxin-signaling F box proteins (AFBs). Phylogenetic studies reveal that these proteins fall into four clades in flowering plants called TIR1, AFB2, AFB4, and AFB6. Genetic studies indicate that members of the TIR1 and AFB2 groups act as positive regulators of auxin signaling. In this report, we demonstrate a unique role for the AFB4 clade. Both AFB4 and AFB5 function as auxin receptors based on in vitro assays. However, unlike other members of the family, loss of AFB4 results in a range of growth defects that are consistent with auxin hypersensitivity, including increased hypocotyl and petiole elongation and increased numbers of lateral roots. Indeed, qRT-PCR experiments show that afb4-2 is hypersensitive to indole-3-acetic acid (IAA) in the hypocotyl, indicating that AFB4 is a negative regulator of auxin response. Furthermore, we show that AFB4 has a particularly important role in the response of seedlings to elevated temperature. Finally, we provide evidence that the AFB4 clade is the major target of the picloram family of auxinic herbicides. These results reveal a previously unknown aspect of auxin receptor function.
authors.affiliation: Section of Cell and Developmental Biology, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Katie
authors.initials: K
authors.lastname: Greenham
authors.forename: Aaron
authors.initials: A
authors.lastname: Santner
authors.forename: Cristina
authors.initials: C
authors.lastname: Castillejo
authors.forename: Sutton
authors.initials: S
authors.lastname: Mooney
authors.forename: Ilkka
authors.initials: I
authors.lastname: Sairanen
authors.forename: Karin
authors.initials: K
authors.lastname: Ljung
authors.forename: Mark
authors.initials: M
authors.lastname: Estelle
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000378:metabolism / Q000502:physiology*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D017931:DNA Primers / Q000235:genetics; D004591:Electrophoresis, Polyacrylamide Gel; D008401:Gas Chromatography-Mass Spectrometry; D018546:Hypocotyl / Q000254:growth & development; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D036226:Seedlings / Q000378:metabolism / Q000502:physiology*; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology*
chemical_list: C559638:AFB4 protein, Arabidopsis; C516178:AFB5 protein, Arabidopsis; D029681:Arabidopsis Proteins; D017931:DNA Primers; D010940:Plant Proteins; D011956:Receptors, Cell Surface; C046131:auxin receptor, plant